we have that \(\mathbf {t}\mathbf {u}\in \mathcal {N}^\ell (\Theta )\)  iff \(\mathbf {t}\)  is a critical point of \(J^\ell _{\mathbf {u}}\) . Note that (REF ) implies that \(u_i\ne 0\)  for all \(i=1,\ldots ,\ell \) . Hence, \((ii)\)  follows from [1]}.
